You start with 0 RP. You increase your multiplayer Tetris Grade by winning versus games across Xbox Live. You decrease your RP by losing versus games across Xbox Live. The way Ranking Points (RP) are awarded is very simple:

As you can see, even if you beat someone ranked many grades above you, you'll only get 10 ranking points. This was intentionally done by the developers, so that people would play with people around their own Grade, so that the skill levels are fair. However, they wanted it so that if a lower-Grade player wanted to play a higher-Grade player, the higher-Grade player would have the incentive to play, because they wouldn't lose an incredible amount of RP if they lost.